Japanese Patent Unexamined Publication No. 2005-084500 discloses a conventional active vibration noise controller that is equipped with multiple speakers as a secondary sound generator, and microphones as an error signal detector, in an enclosed space such as in an automobile cabin; and suppresses noise at a position spaced from the microphones, using a compensating filter to actively reduce noise at a simulated evaluation point.
The conventional apparatus uses multiple speakers 11, 12 as a secondary sound generator, as shown in FIG. 4. The filter coefficient of adaptive filter 14 is successively updated so as to minimize an error signal detected by microphone 13 as an evaluation point, owing to the secondary sound from speaker 11 at the front seat and from speaker 12 at the rear seat, allowing optimal performance of vibration noise suppression to be achieved at an evaluation point.
Further, the filter coefficient of compensating filter 15 is determined according to the ratio of the transmission characteristic from speaker 11 at the front seat to a simulated evaluation point positioned where is spaced from microphone 13; to the transmission characteristic from speaker 12 at the rear seat to the simulated evaluation point. Consequently, at the simulated evaluation point at the rear seat, secondary sound from speaker 11 at the front seat can be cancelled by that from speaker 12 at the rear seat, and thus speaker 11 at the front seat suppresses vibration or noise occurring at the simulated evaluation point at the rear seat.
However, secondary sound supplied from speaker 12 at the rear seat through compensating filter 15 only cancels the effect of an output signal from speaker 11 at the front seat on the simulated evaluation point, at the simulated evaluation point. That is, at the simulated evaluation point, residual vibration noise, namely an error signal, is not detected due to absence of an error signal detector such as a microphone, and thus noise change is not followed at the simulated evaluation point. Consequently, effective noise reduction is not achieved at the simulated evaluation point when the transmission characteristic from the speaker to the simulated evaluation point changes due to changes of the speaker characteristic or to opening/closing of a window.